Correlation Between the Geologic Record and Computed Plate Motions for Central California

We have attempted to correlate geologic events in part of central California with computed motions of oceanic plates relative to cratonic North America. Three composite terranes are considered: (1) the Sierra Nevada, (2) the Coast Ranges northeast of the San Andreas fault, and (3) the Salinian block. In the Sierra Nevada, Jurassic plutonism (ending about 147 m.y. B.P.) and Cretaceous plutonism (120-80 m.y. B.P.) correlate with deduced Farallon-North America (FA:NA) convergence, as does the Nevadan orogeny (158-153 m.y. B.P.). However, the gap in plutonism 146-121 m.y. B.P. outlasted by far an apparent minimum in convergence (145-135 m.y. B.P.). Magmatism ceased in the Sierra and uplift slowed during the Laramide orogeny 75-45 m.y. B.P. when tectonic activity had moved east to the site of the Rockies, presumably because of a low angle of subduction, but volcanisin resumed in the Sierra about 33 m.y. B.P., indicating that a steeper angle was reestablished. Apparently east-west compression slackened when the Pacific-North America (PA:NA) transform regime developed. Basalt was erupted in the southern Sierra 12-3 m.y. B.P., and during this same time, the range started its most rapid rise as a normal-fault-bounded block. The Coast Ranges north-east of the San Andreas fault are underlain by the Franciscan Complex. The older parts of the Complex were probably assembled in a subduction zone far south of their present location, but we believe that the same FA:NA convergence that was cited for Sierran plutonism was responsible, despite the disparity in latitude, as the Farallon plate was probably in contact with most of the western margin of North and South America. A perceived strong pulse in convergence 100-85 m.y. B.P. evidently produced the Coast Range thrust which on geologic grounds alone, most likely originated between 96 and 88 m.y. B.P. Some of the northwesterly transport of Franciscan rocks relative to North America is ascribed to oblique Kula-North America (KU:NA) convergence 85-43 m.y. B.P., during which period the Coast Range thrust was revived about 60 m.y. B.P. The Farallon plate again influenced the California margin after the KuIa plate had moved northward, and FA:NA convergence caused the accretion of the Coastal Belt and King Range terranes of the neo-Franciscan. The advent of Pacific-North America (PA:NA) interaction was marked by local volcanism which advanced from the southeast toward the northwest in consonance with the passage of the Mendocino triple junction (Dickinson and Snyder, 1978). In the Salinian block, plutons 107-82 m.y. old can be ascribed to the same FA:NA convergence as the contemporary Cretaceous plutons of the Sierra Nevada, although the Salinian intrusives originated much farther south. Paleomagnetic results (Champion et al., 1980) from the Salinian-related Pigeon Point Formation, 75-71 (?) m.y. in age, suggest about 2500 km of poleward (essentially coastwise) movement. This postulated large movement during part (or all?) of the last 75 m.y. poses the greatest dilemma in our study. The KuIa is the only known plate whose (computed) obliquely convergent motion is appropriate for the Late Cretaceous-Paleogene part of the migration, and even this motion is inadequate for the estimated transport within the most likely time window. Furthermore, the Farallon plate seems ideally suited for the Laramide orogeny 70-45 m.y. B.P. Both plates could not have been in contact with the same part of the margin at the same time. Oblique convergence of the Kula plate could possibly have caused large coastwise transport and also the Laramide orogeny. The Coast Ranges northeast of the San Andreas fault and the Salinian block both experienced strike slip faulting and en echelon folding correlative with PA:NA interaction, indicating that they were close together by Neogene time. Faulting and folding have been particularly marked since the inception of spreading in the Gulf of California and attachment of westernmost California to the Pacific plate about 4 m.y. B.P. The rise of individual coastal ranges, commencing 3-1 m.y. B.P., may possibly have been caused by slight convergence which appears in computed PA:NA relative motions from 5 m.y. B.P. to the present.